Big problem with my engine 454 gm help

[erick11s]

I have a boat 27 feet 1994 Cobalt 272 sport, Bravo 3 , engine GM 454, with Holly carb dual pump, and Edelbrock manifold. (it doesn't have computer, old version)
PROBLEM: The engine works great in low to medium acceleration, but when going top speed, starts to jump dramatically and loses speed in the range of 4800 RPM 43 miles to 4200 RPM , past 1 minute, it loses RPM and stays at 4200 RPM WITH a RIDICULOUS maximum speed of 30 miles.
I have already done all this:
-Changed the spark plugs (original AC DELCO )
-Changed Spark Plug and coil wires ( 8 mm yellow)
-Changed for a new and complete distributor ( all components are new)
-Changed the gaskets for the manifold (original oem)
-Changed the Gas Pump
-Changed the coil
-Changed the gas filter water separetor.
-Complete service for the carburator.
-checked the gas line (tested with auxiliar gas tank)
-Temperature is OK
-Time is ok. (cheked with the Gun Lamp)

PLEASE HEEEEEEEEELLLLLLLLPPPPPPPPP

ERICK

 

[littlebookworm]

Re: Big problem with my engine 454 gm help

Three things are needed for an engine to run: air, fuel, and spark. Air is pretty straight forward; there's no replacement air filter. You've changed much of the ignition system, including the distributor. That would eliminate the mechanical advance mechanism. You've checked/changed most of the fuel system, including the filters. I'd check two aspects of the fuel system. I'd replace any length of flexible fuel line you have with new E10 rated fuel line. The ethanol in today's gasoline has been known to cause flexible fuel lines to collapse on the inside while under pressure. If that doesn't help, I'd look to the carb itself. Again, ehanol is known to cause clogged jets. That would call for a rebuild. If that doesn't fix it (and I think it will), I would look for a hydraulic lifter collapsing under speed. I've seen that happen with big- block car engines. Marine engines are basically the same inside. Good luck, and let us know what you find. Hy

 

[jtmarten]

Re: Big problem with my engine 454 gm help

What kind of dizzy? How is the advance setup? At what RPM do you achieve full advance?
Did you time it at idle, or at full advance?

If you simply dropped in an automotive dizzy it'll never run right until you setup the advance properly.

 

[Bondo]

Re: Big problem with my engine 454 gm help

Ayuh,... Maybe the Hi-speed jetting is too Lean, or there's Crap in the carb...

Has it Ever run Right with that manifold,+ carb,..??

 

[26aftcab454]

Re: Big problem with my engine 454 gm help

bad valve?

 

[DavidW2009]

Re: Big problem with my engine 454 gm help

Or a broken or weak valve spring.

 

[erick11s]

Re: Big problem with my engine 454 gm help

Three things are needed for an engine to run: air, fuel, and spark. Air is pretty straight forward; there's no replacement air filter. You've changed much of the ignition system, including the distributor. That would eliminate the mechanical advance mechanism. You've checked/changed most of the fuel system, including the filters. I'd check two aspects of the fuel system. I'd replace any length of flexible fuel line you have with new E10 rated fuel line. The ethanol in today's gasoline has been known to cause flexible fuel lines to collapse on the inside while under pressure. If that doesn't help, I'd look to the carb itself. Again, ehanol is known to cause clogged jets. That would call for a rebuild. If that doesn't fix it (and I think it will), I would look for a hydraulic lifter collapsing under speed. I've seen that happen with big- block car engines. Marine engines are basically the same inside. Good luck, and let us know what you find. Hy

Dear Friend Thanks for you reply.....the situation at this moment is:

I CHECKED the fuel lines and put A new fuel filter, also reviewed the springs and are double and look good ... the 170 is in compression on all cylinders for this we think the valves be well, the new is WE CHEKEDthat the spark plugs only had 1 hour of use and were all black, we assume that the problem remains l the carburetor? or the chain of the time...... I need opinions .... please and thank you for your comments.

Erick

 

[buzzm19]

Re: Big problem with my engine 454 gm help

Erick, I would check your power valves in your carb plates, don't know if you have two or only one in the front. If it poped through the carb you could destroy those valves or the diaphragms could be weak and staying open or a piece of dirt. I know holleys and they need to have a filter no farther than 6" from the carb, hope this helps. Buz